Michael A. Covington    Michael A. Covington, Ph.D.
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Replacing a tuning-eye tube
Two-transistor static protection for shortwave antenna input

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Two-transistor overvoltage protection for shortwave radio antenna input

A two-transistor bidirectional Zener clamp makes a very good "static protector" for the input of a shortwave radio. It protects against static electricity and also voltages induced by thunderstorms in the area (which can be hundreds of volts even if not very close to a lightning strike). I added one to my antenna tuner, so its complete circuit is now what is shown below. The red components are added to the classic antenna tuner circuit.


Unlike the traditional pair of diodes, the bidirectional clamp doesn't turn on until it reaches about 8 volts; that means it won't be affected by strong signals from nearby AM stations or power lines. But, unlike a Zener diode, it has very little leakage and little capacitance. The MPF102 input transistor in my radio can tolerate 25 volts, so an 8-volt clamp is sufficient, I think.

You can see that I also made the resistor termination switchable. The reason? With the resistors in place (switch closed), the antenna picks up less noise but is also less tunable; it loses a lot of sensitivity at frequencies other than the one it is cut for. With the resistors removed (switch open), it is much more tunable. For more about this, click here.

Here's what the two-transistor clamp looks like, assembled:


And here's what my ancient Tektronix transistor curve tracer tells me about the threshold voltages (2 volts per division):


Note: I would consider selling this instrument (local pickup in Athens, Georgia, only; it weighs 75 pounds). It is fully functional but hasn't had any substantial maintenance in several years. Contact me if you want to make an offer.

Flu express

This notebook entry is the only thing I've produced today (for anyone) because I'm taking a sick day. Last night I appeared to have the flu, but only for eight hours or so, though there is a lingering slight fever. I've had this experience in several recent years; maybe the vaccine enables me to fight off the viruses with only a short battle. Or maybe it's something else.


Replacing a tuning-eye tube


Happy new month! And a blessed First Sunday in Advent to all.

If you didn't see yesterday's entry about the noble art of electronic repair, and why the repair business is a lot like the insurance business, click here and read it. Then proceed...

On Saturday I did something in my workshop that is rarely done in this century. I replaced a tuning-eye tube.

As you can see, it made a dramatic difference. But what is this contraption?

It's a vacuum tube that glows green on one end, and part of the green glow is blocked off, depending on a signal voltage applied to it. Above, you see it as the display device on my somewhat souped-up Heathkit capacitor tester.


Back in the 1930s, radio manufacturers wanted to include a meter in the radio, to help people tune each station exactly. (Tune for the highest meter reading.) But meters are expensive, and there would also need to be a tube to amplify the signal for the meter. Further, the purpose of such a meter is just to find the maximum — it doesn't have to have a calibrated scale.

So someone had the idea of building an indicator device into the amplifying tube itself. The "tuning eye" is what resulted. It looks rather like an eye, hence the name.

By the 1960s, tuning eyes were rare; everybody got good local radio reception and didn't feel the need for a tuning aid. Shortwave radios for serious listeners had meters, not tuning eyes. So the technology was almost obsolete...

But tuning eyes hung on in a few kinds of test equipment, notably capacitor testers. My Heathkit IT-11 has one. And what I didn't know, when I bought that instrument about a quarter century ago, was that its tuning eye was on its last legs. It was hard to see.

I can put up with a dim display for 25 years, but not forever. So I replaced the tube. Here are the old and new ones, for comparison. The phosphor in the old one has deteriorated and turned brown. The new one (about 60 years old, but never used, and fresh in its RCA box from an eBay vendor) is tremendously brighter.


There is an excellent video about eye tubes at this link, for Patreon subscribers to "Mr. Carlson's Lab." My recollection is that you can subscribe for one month for $2, well worth it.

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